1. Field of the Invention
The present invention relates to a fuel pump that comprises a casing and a substantially disc-shaped impeller rotating within the casing.
2. Description of the Related Art
Fuel pumps that function as devices for supplying fuel within a fuel tank to an internal combustion engine (e.g., the engine of an automobile) are known to the art. This type of fuel pump usually comprises a motor portion and a pump portion. The pump portion comprises a casing and a substantially disc-shaped impeller housed so as to be capable of rotating within the casing. A first group of concavities is formed in a ring shape in the intake side face of the impeller. The first group of concavities is formed concentrically with the impeller along an outer peripheral portion of this impeller. A second group of concavities is formed in the discharge side face of the impeller at a position corresponding to the first group of concavities formed at the intake side. The first group of concavities in the intake side face of the impeller communicates with the second group of concavities formed in the discharge side face thereof.
A first groove is formed in an inner face of the casing that faces the intake side face of the impeller. The first groove is formed in an area that faces the area of the impeller in which the first group of concavities is formed. A second groove is formed in an inner face of the casing that faces the discharge side face of the impeller. The second groove is formed in an area that faces the area of the impeller in which the second group of concavities is formed. The grooves extend along the direction of rotation of the impeller from upper flow ends to lower flow ends, respectively. The upper flow end of the first groove at the intake side communicates with the fuel tank via an intake hole. The lower flow end of the second groove at the discharge side communicates with the motor portion via a discharge hole.
In this fuel pump, fuel is drawn into the casing through the intake hole when the impeller rotates. The fuel that has been drawn in is led along the groups of concavities of the impeller and the grooves. The rotation of the impeller exerts a centrifugal force upon the fuel that has been drawn into the casing. The centrifugal force of the impeller increases the pressure of the fuel that has been drawn into the casing while this fuel flows downstream along the grooves. The fuel that has reached the lower flow end of the second groove is discharged to the exterior of the casing from the discharge hole.
In this type of fuel pump, the fuel that has been drawn into the casing is violently agitated by the concavities, and consequently the velocity of the fuel increases abruptly while the pressure thereof falls abruptly. Vapor forms within the fuel when the pressure of the fuel falls. In particular, when the air temperature increases the saturated vapor pressure increases, and consequently, vapor can readily form. The formation of a large amount of vapor within the fuel can cause a vapor lock. The performance of the fuel pump is thereby reduced. A fuel pump capable of effectively preventing vapor locks has been proposed to deal with this problem (e.g., Japanese Laid-open Patent Publications No. 60-113088 and No. 60-219495).
This known fuel pump comprises a first impeller, and a vapor-separating impeller that has a narrow diameter and that is disposed at the upper flow side from the first impeller. The two impellers are coaxial. Any abrupt change in the pressure of the fuel is reduced by the fuel being drawn in by the vapor-separating impeller. The formation of vapor is thus reduced. Furthermore, the centrifugal force generated by the rotation of the vapor-separating impeller causes the fuel to flow toward the outer circumferential region thereof while the vapor that formed when the fuel was being drawn in collects at an inner circumferential region thereof. The fuel and the vapor are thus separated. The separated vapor is transported toward a vapor jet formed in a region inwards from a fuel discharge passage in the radial direction. A vapor lock can be prevented by removing the vapor from the fuel in this manner.